The present invention relates to a cascaded narrowband filter using an acousto-optic tunable filter (AOTF) for wide band discrimination and birefringent filters for narrow band discrimination.
Most of the optical channel monitor (OCM) use gratings as dispersion elements to separate the wavelengths of light contained in the optical beam being examined. An optical spectrum of the beam is usually obtained by mechanically rotating a grating such that light diffracted at various wavelength-dependent angles is incident on a photodetector. Others solution implement a photodetector array that is also called a spectrometer.
An alternative method to obtaining an optical spectrum relies on the use of an acousto-optic tunable filter (AOTF). An AOTF is controlled by a tunable RF source to adjust the refraction of light in accordance with wavelength. The AOTF has many advantages over gratings including scan speed and ability to be integrated into a small size package. Unfortunately, AOTFs are limited by their poor spectrum resolution and accuracy. For example, the typical resolution of current AOTFs is about 100 GHz (60 GHz at full-width half maximum FWHM) and the temperature shift is about 100 pm/xc2x0 C. In monitoring optical telecommunication systems with channels defined in accordance with the wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) standards such low resolution can be used for determining channel intensity only.
The prior art teaches several approaches to improving the resolution and accuracy of AOTFs. For example, Paek describes a grating assisted acousto-optic tunable filter (AOTF) and method in U.S. Pat. No. 5,946,128. The filter combines a diffraction grating with the AOTF for resolving channel crosstalk issues and to provide rapid tunability. In addition, the narrower bandwidth of Paek""s filter permits a larger number of wavelength channels within the passband of an Erbium Doped Fiber Amplifier (EDFA).
In another approach, an AOTF is used in combination with a Fabry-Perot interferometer to achieve higher resolution spectroscopy. Corresponding teaching is provided, among other, by I. C. Chang, et al., xe2x80x9cAcousto-Optic Tunable Filters for High Resolution Spectral Analysisxe2x80x9d, Proceedings of the SPIE, Vol. 268, pp. 167-70 (Feb. 10-11, 1981) and D. P. Baldwin, et al., xe2x80x9cHigh-Resolution Spectroscopy Using an Acousto-Optic Tunable Filter and a Fiber-Optic Fabry-Perot Interferometerxe2x80x9d, Applied Spectroscopy, Vol. 50, No. 4, pp. 498-503 (April 1996).
U.S. Pat. Nos. 6,330,255 and 6,330,254 both to Hung teach an integrated optic device for optical wavelength selection which uses an AOTF as a broad band tunable filter and is followed by a narrowband tunable filter. Hung teaches that the narrow band tunable filter can incorporate a Mach-Zhender interferometer or other narrowband filters, including polarization-type filters. The system becomes complicated because two tunable filters are needed. Consequently, the wavelength and bandwidth measurement accuracy are sacrificed.
The prior art teaches the use of birefringent filters in a number of applications. For example, Jingshan Wang, et al. teach the use of a birefringent filter system in xe2x80x9cOptical Design of a Near-Infrared Birefringent Filter System and Measurement on Birefringence Index of Calcitexe2x80x9d, Proceedings of the SPIE, Vol. 4093, pp. 481-489 (2000) to observe the solar spectrum. The system uses a pre-filter, an analyzer and the birefringent filter as a final narrowband filter. In WO 01/84196 Li teaches the use of a wavelength filter comprised of birefringent waveplates within an optical interleaver. Imaki Masao, et al. in WO 01/57487 also teach the use of a birefringent crystal as a wavelength filter for monitoring the wavelength of a laser beam. The wavelength detection circuit uses two photodetectors, one to sense the s-polarized light and the other to sense the p-polarized light.
Although the prior art teaches optical spectral analyzers and various wavelength filtering techniques, it does not provide a reliable, small and fast optical channel monitor (OCM), which can be used in wavelength monitoring applications. In particular, it would be an advance in the art to design an OCM that can be used in channel monitoring in optical fiber communication systems. It would be particularly advantageous to provide an optical channel monitor OCM that can measure a center frequency of a channel, its bandwidth and its intensity.
In view of the above, it is an object of the present invention to provide a novel cascaded filter that can be used for wavelength monitoring. In particular, it is an object of the invention to provide an optical channel monitor employing such cascaded filter design for monitoring wavelength channels in communication systems by measuring their center frequencies, bandwidths and its intensity. These and other object and advantages will become apparent upon reading the detailed description.
The objects and advantages of the invention are achieved by an optical channel monitor (OCM) for analyzing an incident light carrying a number of narrow band signal channels. In general, the signal channels can represent any narrow portions of the spectral band spanned by the incident light. For example, the incident light can contain signal channels that are Wavelength Division Multiplexed (WDM) channels or Dense Wavelength Division Multiplexed (DWDM) channels in an optical communication network.
The OCM has an acousto-optic tunable filter (AOTF) for receiving the incident light and refracting from it a refracted light such that the refracted light contains one of the narrow band signal channels or a test channel with a center frequency xcexd0. A first birefringent element is provided for filtering from the refracted light a first polarized light and a second polarized light orthogonal to the first polarized light. The transmission curves for the first polarized light and of the second polarized light are generally periodic and out of phase with each other. The transmission for both first and second polarized light varies between a maximum and minimum transmission level. Therefore, the intensities of the first and second polarized light vary between corresponding maximum and minimum values. In accordance with the invention, the transmission curves are engineered such that the transmissions of the first and second polarized light are substantially equal at the center frequency xcexd0 of the test channel. In other words, the transmission curves have equal transmission values at the center frequency xcexd0.
The OCM has a second birefringent element for filtering from the first polarized light a first polarized portion and a second polarized portion. Once again, the first and second portions are orthogonal to each other. The transmission curves of the second birefringent element are set such that the transmissions of the first and second polarized portions are substantially equal at a first offset xcex41xcexd from the center frequency xcexd0. A set of photodetectors is used for measuring the intensities of the first polarized light and the intensities of the first and second filtered portions. An analysis unit, e.g., a processor-based computing unit, is connected to the photodetectors to derive from the intensities the center frequency xcexd0 and the bandwidth xcex94xcexd0 of the test channel.
The OCM uses polarization separators, e.g., polarization beam splitters or polarization walk-off elements, for separating and directing light based on its polarization. A first polarization separator is used for directing the first polarized light to the second birefringent element. A second polarization separator is placed after the second birefringent element for directing the first and second polarized portions to a first pair of photodetectors, i.e., first polarized portion to one and the second polarized portion to the other photodetector of the first pair.
In a preferred embodiment of the OCM a third birefringent element is provided for filtering from the second polarized light a third and a fourth polarized portion. The polarizations of these portions are orthogonal to each other. The third birefringent element is designed such that the transmissions of the first and fourth polarized-portions are substantially equal at a second offset xcex42xcexd from the center frequency xcexd0. A third polarization separator is provided in this embodiment for directing the third and fourth polarized portions to a second pair of photodetectors.
In another embodiment no third birefringent element is used. Instead, the first polarization separator directs the second polarized light to a single photodetector. In this embodiment less intensity data is provided from the photodetectors to the analysis unit for determining the center frequency xcexd0 and the bandwidth xcex94xcexd0 of the test channel.
The principles of the invention can be used in an optical filter for filtering a test channel from among WDM or DWMD channels. In some embodiments more than one signal channel can be contained in the refracted light. In those embodiments additional polarization separators and birefringent elements are required to analyze the center frequencies and bandwidths of the signal channels.
The design of the OCM or filter can include additional optics to focus and/or collimate the refracted light and/or the polarized portions at any step of the wavelength monitoring or filtering process. Any other elements required for stable operation, e.g., temperature control mechanisms to maintain the birefringent elements at a constant temperature, can be integrated into OCMs or filters in accordance with the invention. The details of the invention are discussed below with reference to the attached drawing figures.